The establishment of segmentation in the Drosophila leg

Segmentation is a developmental mechanism that subdivides a tissue into rep eating functional units, which can then be further elaborated upon during d evelopment. In contrast to embryonic segmentation, Drosophila leg segmentat ion occurs in a tissue that is rapidly growing in size and thus segmentatio n must be coordinated with tissue growth. I demonstrate that segmentation o f the Drosophila leg, as assayed by expression of the key regulators of seg mentation, the Notch ligands and fringe, occurs progressively and I define the sequence in which the initial segmental subdivisions arise. I further d emonstrate that the proximal-distal patterning genes homothorax and dachshu nd are positively required, while Distal-less is unexpectedly negatively re quired, to establish the segmental pattern of Notch ligand and fringe expre ssion. Two Serrate enhancers that respond to regulation by dachshund are al so identified. Together, these studies provide evidence that distinct combi nations of the proximal-distal patterning genes independently regulate each segmental ring of Notch ligand and fringe expression and that this regulat ion occurs through distinct enhancers. These studies thus provide a molecul ar framework for understanding how segmentation during tissue growth is acc omplished.